1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image. In addition, the present invention also relates to a developer, a toner container, and an image forming apparatus using the toner.
2. Discussion of the Background
Electrophotographic image forming methods have been used for various fields. Electrophotographic image forming methods typically include the following processes.
(1) charging the surface of an image bearing member such as photoreceptors (charging process):
(2) irradiating the charged image bearing member with light to form an electrostatic latent image on the image bearing member (light irradiating process);
(3) developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member (developing process);
(4) transferring the toner image onto a receiving material (transfer process); and
(5) fixing the toner image to the receiving material upon application of heat and pressure thereto (fixing process).
With respect to the fixing method, heat roller fixing methods in which a toner image on a receiving material is pressure-contacted with a heat roller to be fixed thereon are broadly used. Heat roller fixing devices typically include a heat roller and a pressure roller, and a receiving material bearing a toner image thereon is passed through the nip between the heat roller and the pressure roller to be melted, resulting in fixation of the toner image on the receiving material.
When a heat fixing method is used, the heating temperature is preferably as low as possible to save energy. However, when a binder resin having too low a thermal property (such as the melting point and the glass transition temperature) is used for a toner so that the toner can be used for such a low temperature heat fixing method, the high temperature preservability of the toner deteriorates, thereby causing a blocking problem in that the toner is blocked when preserved at a relatively high temperature. In order to impart a good combination of low temperature fixability and high temperature preservability to a toner, a polyester resin is preferably used as a binder resin of the toner. Because of having relatively low viscosity and high elasticity, polyester resins have a relatively good combination of low temperature fixability and high temperature preservability compared with vinyl resins.
Polyester resins prepared by using a bisphenol compound as a polyhydric alcohol have been broadly used as binder resins of toner. However, it is proposed to use an aliphatic or alicyclic polyhydric alcohol to prepare a polyester resin having good flexibility. This is because aliphatic or alicyclic polyhydric alcohols have molecular structures with high flexibility whereas bisphenol compounds have high rigidity.
On the other hand, in order to fix a toner image at a relatively low fixing temperature, it is important that the toner image is firmly adhered to the receiving material at a relatively low fixing temperature so as to have good rubbing resistance (i.e., good durability). In this regard, aliphatic or alicyclic polyhydric alcohols have relatively low molecular weights and therefore the number of ester groups included in the resultant polyester resins is greater than that of the ester groups included in the polyester resins prepared using bisphenol compounds. Therefore, the interaction between the toner and the hydroxyl group included in the receiving material (such as paper) can be increased, resulting in improvement of the durability of the toner image.
In attempting to impart a good combination of low temperature fixability and preservability, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2002-091082 discloses a toner including, as a binder resin, a polyester resin obtained from a polyester component (A) which is prepared by subjecting a diol and a dicarboxylic acid to condensation polymerization and another polyester component (B) which is prepared by subjecting a branched diol (preferably an aliphatic or alicyclic diol) and a dicarboxylic acid (preferably an acid having a terephthalic structure) to condensation polymerization.
In addition, JP-A2002-055483 discloses a toner including, as a binder resin, a polyester resin which is obtained from a dicarboxylic acid component and a diol component, wherein the diol component includes a branched diol in an amount of from 20 to 90% by mole and the dicarboxylic acid component includes a carboxylic acid having a terephthalic structure in an amount of from 80 to 99.9% by mole and another carboxylic acid having an o-phthalic acid structure in an amount of from 20 to 0.1% by mole.
Further, JP-A 08-036274 discloses a toner which includes a polyester resin obtained from (a) an acid component including terephthalic acid as a main component, (b) a diol component including as a main component a linear alkylene glycol having 2 to 8 carbon atoms, and (c) a polycarboxylic acid having three or more carboxyl groups and/or a polyhydric alcohol having three or more hydroxyl groups, wherein the ratio of terephthalic acid and the linear alkylene glycol to all the monomers is not less than 50% by mole and the ratio of the component (c) to all the acid components is not greater than 3% by mole. It is described therein that the toner has relatively high hot offset temperature, wide fixable temperature range and good blocking resistance compared to conventional toners.
Japanese patent No. 3128907 (i.e., JP-A 05-165252) discloses a toner including a polyester resin which is prepared by reacting a polybasic acid component including terephthalic acid as a main component and a polyhydric alcohol component including (A) 1,4-cyclohexanedimethanol and (B) a glycol having a specific formula, wherein the molar ratio (A)/(B) is from 35/65 to 65/35, the molar ratio of total of the components (A) and (B) in the polyhydric alcohol component is not less than 90%, and the polyester resin has a glass transition temperature of from 55 to 75° C., a weight average molecular weight of from 5,000 to 20,000, a melt viscosity of from 104 to 106 poise, and a softening point of from 90 to 120° C. (measured by a ring and ball method). It is described therein that the toner has a good combination of fixability, blocking resistance, offset resistance and color property.
JP-A 05-100480 discloses a toner including a polyester resin which has a specific gravity of not less than 1.3 and which is obtained from a polycarboxylic acid component including terephthalic acid, isophthalic acid and a polycarboxylic acid having a sodium sulfonate group and a polyhydric alcohol component including ethylene glycol and/or its derivatives in an amount of not less than 90% by mole. It is described therein that the toner has a good combination of blocking resistance (i.e., preservability) and resistance to plasticizers included in polyvinyl chloride.
Japanese patent No. 2704282 discloses a toner including a polyester resin which is obtained by subjecting a monomer composition, which includes a monomer component having three or more functional groups, an aromatic dicarboxylic acid component, and an aliphatic dihydric alcohol component including a branched dihydric alcohol in an amount of not less than 50% by mole based on the total of the aliphatic dihydric alcohol component, to condensation polymerization. It is described therein that the toner has a good combination of fixing property, offset resistance, and releasability from fixing members.
As mentioned above, the toners including, as a binder resin, a polyester resin, which includes an aliphatic or alicyclic polyhydric alcohol component, have a good low temperature fixability. However, these toners merely have an improved thermal property and the high temperature preservability thereof is hardly improved or is rather deteriorated. In addition, these toners include an organic tin compound which is used as a catalyst and which is not friendly to the environment. Namely, it is necessary to replace such a catalyst with another catalyst such as titanium-type catalysts, bismuth-type catalysts, and inorganic tin catalysts.
Because of these reasons, a need exists for a toner having a good combination of low temperature fixability and high temperature preservability.